The present invention relates to broadcast transmission systems and is particularly directed to compensation of distortion within a digital transmission system, such as a digital TV (xe2x80x9cDTVxe2x80x9d) transmission system.
A high-speed broadcast transmission system such as a DTV broadcast system includes components that distort an information signal away from intended values. Specifically, the system includes a power amplifier that imposes non-linear distortion upon the signal, as the signal is amplified. Also, the broadcast transmission system includes filters, such as band-limiting filters, that impose linear distortion upon the information signal as the signal is filtered.
As a result of such distortions within the transmission system, instantaneous amplitude and phase variations (AM/AM, AM/PM) and frequency dependent amplitude and phase variations (frequency response and group delay) occur. It is to be appreciated that within a phase-amplitude modulated system, amplitude and phase integrity of the system must be preserved for optimum system performance.
Traditional equalization for television systems has been accomplished by analog, pre-distortion equalizers and correctors that are static (non-adaptive). Such equalizers and correctors require factory adjustments to provide a desired amount of pre-distortion (pre-equalization). Aging of the equalizers and correctors, and temperature change cause drift in the amount of pre-distortion that is imposed by the equalizers and correctors. Occasional field adjustments are required.
Digital signal processing techniques provide improved performance of the pre-distortion of the information signal. Specifically, digital signal processing can be used in an adaptive correction and equalization approach. Such an adaptive approach can eliminate the factory and field adjustments.
It is known to perform adaptive correction of a signal within a signal stream proceeding toward an antenna. However, in a relatively fast data system, the correction requires a relatively large amount of processing in a short period of time. In one known technique, all of the distortion (i.e., linear and non-linear) is corrected in a single step.
In another technique, the correction for the distortion imposed within the system is done component by component proceeding in a direction toward the antenna. Specifically, for each component, the signal that is output from that component is monitored to determine the amount of distortion imposed by that component. A correction is then developed for that component. Subsequently, the next subsequent component is monitored to develop the correction for that component. However, such a technique is time consuming and is often unsuitable for high data rate streams. Further, within such a system, amplitude or group delay variations over frequency can be misinterpreted to be instantaneous amplitude and phase variations.
Thus, there is a need for a high-speed technique for adaptive correction of linear and non-linear distortion within a digital broadcast transmission system.
In accordance with one aspect, the present invention provides a transmission system for broadcasting an information signal. The system includes a first plurality of components arranged in a sequence and including at least one amplifier. Each of the first plurality of components performs a function on the information signal, and each of the first plurality of components subjects the information signal to distortion shifts away from intended values. The system includes a second plurality of components for modifying the information signal to compensate for the distortion shifts imposed by the first plurality of components. The second plurality of components is located upstream of the first plurality of components. The second plurality of components is arranged in a sequence to modify the information signal to compensate for the distortions in an order inverse to the occurrence of the distortions.
In accordance with another aspect, the present invention provides a transmission system that includes a first plurality of components in a sequential arrangement. Each of the first plurality of components performs a function on the information signal. A first component of the first plurality of components subjects the information signal to linear distortion shifts away from intended values. A second component of the first plurality of components subjects the information signal to non-linear distortion shifts away from intended values. The system includes a second plurality of components that is located upstream of the first plurality of components. The second plurality of components modifies the information signal to compensate for the distortion shifts imposed by the first plurality of components. A first component of the first plurality of components modifies the information signal to compensate for the linear distortion. A second component of the second plurality of components modifies the information signal to compensate for the non-linear distortion. The first and second components of the second plurality of components are arranged in a sequence to modify the information signal to compensate for the distortions in an order inverse to the occurrence of the distortions.
In accordance with yet another aspect, the present invention provides a transmission system for broadcasting an information signal and having a signal path along which the information signal proceeds toward an antenna. Within the system, a first component is located on the signal path and performs a function on the information signal. The first component subjects the information signal to non-linear distortion shifts away from intended values. A second component is located on the signal path and performs a function on the information signal. The second component subjects the information signal to linear distortion shifts away from intended values. A third component is located on the signal path and performs a function on the information signal. The third component subjects the information signal to linear distortion shifts away from intended values. The second and third components are grouped together either upstream or downstream of the first component along the signal path. A fourth component is located on the signal path and modifies the information signal to compensate for the non-linear distortion imposed by the first component. A fifth component is located on the signal path and modifies the information signal to compensate for the linear distortion imposed by the second component. A sixth component is located on the signal path and modifies the information signal to compensate for the linear distortion imposed by the third component. The fifth and sixth components are grouped together either upstream or downstream of the fourth component along the signal path. The upstream/downstream location of the fifth and sixth components with respect to the fourth component is opposite to the upstream/downstream location of the second and third components with respect to the first component.
In accordance with still another aspect, the present invention provides a distortion compensation arrangement for a radio frequency transmitter system. The system includes an input circuit for processing digital signals to be transmitted, a digital-to-analog converter for converting the digital signals into analog form, and an up converter for modulating a radio frequency carrier by the analog signals. The system also includes at least one radio frequency filter circuit, and at least one radio frequency amplifier circuit. The filter and amplifier circuits introduce linear and non-linear distortion into the modulated radio frequency carrier. The arrangement includes adaptive digital signal distortion compensation circuitry that is connected between the input circuit and the digital-to-analog converter for processing the digital signals to be applied to the digital-to-analog converter. The adaptive digital signal distortion compensation circuitry is responsive to output signals from the radio frequency filter and amplifier circuits for modifying the digital signals to provide linear and non-linear compensation to the digital signals. The compensation sequence applied to the digital signals is such that it is inverse to the order in which the filter and amplifier circuits are connected.
In accordance with yet a still further aspect, the present invention provides a digital television radio frequency transmitter system. An input circuit processes digital television signals to be transmitted. A digital-to-analog converter converts the digital television signals into analog form. An up converter modulates a radio frequency carrier by the television analog signals. The system includes at least one radio frequency filter circuit and at least one radio frequency amplifier circuit. The filter and amplifier circuits introduce linear and non-linear distortion into the modulated radio frequency carrier television signals. A down converter receives output radio frequency carrier television signals from the filter and amplifier circuits for down converting the output signals. An analog-to-digital converter converts the down converted analog television signals from the filter and amplifier circuits into digital form. Adaptive digital signal distortion compensation circuitry is connected between the input circuit and the digital-to-analog converter for processing the digital signals to be applied to the digital-to-analog converter. The adaptive digital signal distortion compensation circuitry is responsive to the digital signals from the analog-to-digital converter corresponding to the outputs of the filter and amplifier circuits for modifying the digital television signals to provide linear and non-linear compensation to the digital television signals. The compensation sequence applied to the digital television signals is such that it is inverse to the order in which the filter and amplifier circuits are connected.